The present invention relates to a reduction of encrustation of organic and inorganic substances on the inner surfaces of crystallization or reaction containers, for example adipic acid crusts on the lateral walls of large crystallizers.
The use of pulsed ultrasonic technology for scale prevention in heat exchangers is known, for example, from the article in the journal series xe2x80x9cSanitxc3xa4r- und Heizungstechnik 6/1999xe2x80x9d by V. Podolyak, U. Schumucker and S. Sperling, Fraunhofer Institut, Magdeburg. The paper xe2x80x9cUltraschall: kostensparend und umweltfreundlichxe2x80x9d reveals that pulsed ultrasonic oscillations are used for preventing deposits on heat exchangers. Pulsed ultrasound apparatuses have small external dimensions and require very little electrical energy. The pulse frequency can be regulated in a simple manner, the frequency of the natural oscillations of the ultrasound converter being about 20 kHz. An ultrasound amplitude amplifier is usually fastened to the wall of a heat exchanger to be protected from scale or rust, or specially designed emitters are immersed directly in the liquid. The correct point or the correct points for optimum energy transmission must be found. The efficiency of the pulsed ultrasound is determined mainly by the choice of the points of incidence.
EP-0 584 685 A2 relates to a reactor for carrying out chemical reactions. At least three sonotrodes are arranged on the base of the reactor and at least six sonotrodes are arranged on the lateral surface. The sonotrodes can be integrated both in the base and in the lateral surface of the reactor, which may be designed with either a single wall or a double wall.
U.S. Pat. No. 5,471,001 relates to a crystallization process for adipic acid, an aqueous suspension which contains dissolved adipic acid crystals being subjected to low-intensity ultrasonic excitation while the aqueous solution is cooled and/or while the water content of the aqueous solution is reduced. The intensity of the ultrasonic oscillation is in the frequency range from 20 to 100 kHz, the aqueous solution passing the sonotrodes with a time interval of 5 to 20 seconds as a result of a stirring element projecting into the interior of the reactor.
It is an object of the present invention to reduce the adhesion of solid or crystals to the walls of containers and crystallizers to such an extent that the growth of a solid layer is effectively prevented.
We have found that this object is achieved, according to the invention, by a process for avoiding encrustation in reactors or crystallizers, such as industrial crystallizers for suspensions, on whose walls and in the base region points of incidence for the excitation of ultrasonic oscillations by oscillation exciters, for example sonotrodes, are provided, which are connected to electromechanical converters and whose ultrasonic frequency is generated in each case by means of a high-frequency generator. The excitation of ultrasonic oscillations is effected as a rule in the region above and/or below a suspension level of the container content.
As a result of exciting ultrasonic oscillations, it has surprisingly been found that encrustation can be avoided in the region of the excitation zone. The optimum process parameters for avoiding the build-up of a crust from suspensions or solutions on container walls can be established through the choice of the ultrasonic frequency and the oscillation amplitude and by the number and position of the sonotrodes and the time of action of the oscillations.
The process can be used both in batch apparatuses and in continuously operated apparatuses. The thermostating of the suspensions or solutions can be effected by means of external or internal heat exchangers, by means of thermostating circulations arranged on the container wall or by adjusting the pressure during the evaporation of the solvent under reduced pressure.
In a preferred embodiment of the process proposed according to the invention, the oscillation frequency which the oscillation transmitters (sonotrodes) have at the respective points of incidence in the crystallizer or a container may be from 16 to 100 kHz. However, oscillation excitation outside the ultrasound range, for example with frequencies in the range from 100 Hz to 16 kHz is also possible. The zone of incidence for the ultrasonic oscillations is advantageously formed by a segment of the lateral wall of the crystallizer. Thus, ultrasonic oscillations can be applied uniformly to the suspension so that in particular that edge region of the crystallizer or of the container which is wet by the suspension is subjected to ultrasonic oscillations. The sonotrodes are therefore arranged in particular in an annular manner around the crystallizer; they may furthermore particularly advantageously be arranged in a plurality of planes arranged in an annular manner one above the other. It should be ensured that the distance between the sonotrodes or between pairs of sonotrodes is a nonintegral multiple of half the wavelength, calculated from the excitation frequency and the velocity of sound for the container or reactor material. This serves for avoiding fixed nodes on the container or reactor wall.
Furthermore, the oscillation exciter may also be operated in a periodic manner, i.e. oscillations are excited at certain time intervals with a certain period, in each case either only one oscillation exciter operating or a plurality thereof operating simultaneously. The amplitudes of the oscillation exciters are advantageously from 0.1 to 100 xcexcm. In the case of the excitation of ultrasonic oscillations, the power consumptions of the high-frequency generator is from 100 to 10,000 watts per sonotrode.
In the apparatus, likewise disclosed, for avoiding encrustation on crystallizers for suspensions, points of incidence for ultrasonic oscillations applied by sonotrodes are provided in each case on the crystallizer walls, the ultrasonic frequency being generated at high-frequency generators, and sonotrodes being arranged in a region of the suspension level in the crystallizer or in the container, surrounding the crystallizer or the container in an annular manner.
The invention is illustrated in more detail below with reference to a drawing.